The generation of chlorine or other halogens by electrolysis of an aqueous halide such as hydrochloric acid and/or alkali metal chloride or other corresponding electrolysable halide has been known for a long time. Such electrolysis is usually in a cell in which the anode and the cathode are separated by an ion permeable membrane or diaphragm. In cells having a liquid permeable diaphragm, the alkali metal chloride is circulated through the anolyte chamber and a portion thereof flows through the diaphragm into the catholyte. When alkali metal chloride is electrolyzed, chlorine is evolved at the anode and alkali which may be alkali metal carbonate or bicarbonate, but is more commonly an alkali metal hydroxide solution, is formed at the cathode.
This alkali solution also contains an alkali metal chloride which must be separated from the alkali in a subsequent operation. The alkali solution is relatively dilute, rarely in excess of 12-15% alkali by weight, and since commercial concentrations of sodium hydroxide are normally about 50% or higher by weight, the water in the dilute solution has to be evaporated to achieve this concentration.
When a separator such as an ion exchange membrane is used in a cell to electrolyze a sodium chloride brine, the electrochemical products will normally be gaseous chlorine and an aqueous solution containing sodium hydroxide. The use of a substantially liquid impermeable cation exchange membrane has become the preferred membrane where, for example, a high purity, a lower sodium chloride content, high sodium hydroxide product is desired. It has been found to be more convenient to fabricate ion exchange type electrochemical cells from relatively flat or planar sheets for ion exchange membrane, such as disclosed in U.S. Pat. No. 4,668,371, rather than to interweave the membrane between the anode and cathode within the older finger-like cells used with asbestos diaphragms.
In narrow gap or zero gap electrolysis, the passage of current from one electrode to an opposite electrode takes place only through the ionically-permeable separator, which is the ionic selective and ionic conductive membrane. Current flows from the surface of one separator to the surface of the separator of an adjoining cell only by electronic conductivity (i.e., by the current feeder grids and their associated connections or bipolar separators), then flows ionically to the opposite surface of the separator.
One of the problems which is encountered with these narrow gap or zero gap cells is overcompression which physically damages the membrane. U.S. Pat. Nos. 4,444,632 and 4,693,797 disclose the use of mattresses for overcoming some of the problems resulting from overcompression. However, the prior art does not provide a means for selecting a mattress material for use in large cells and mattresses that compensates for dimensional tolerances of the electrode to electrode spacing of filter press cells. The teachings of small cells (generally having a membrane area of about 12 to 18 sq. ft.) cannot be used effectively for selecting mattresses for large cells.
The essential requirements for a mattress in narrow gap or zero gap cells is to 1) provide sufficient resiliency or springiness so as to maintain all of the components in the cell in uniform compression, 2) conduct the electrical current from the electrode current collector to the electrode, 3) accomplish 1) and 2) so as to achieve a voltage improvement without damage to the membrane and, 4) be self adjusting so as to obtain good and uniform contact distribution over the entire surface of the electrode.
U.S. Pat. No. 4,444,632 discloses a typical small non-pressurized electrolysis cell comprising a cell housing containing at least one set of gas and electrolyte permeable electrodes respectively, an anode and a cathode separated by an ion permeable diaphragm or membrane, at least one of the electrodes is pressed against the diaphragm or membrane by a mattress comprising an open structure resiliently compressible layer co-extensive with the electrode surface. The mattress is compressible against the membrane while exerting an elastic reaction force onto the electrode in contact with the membrane at a plurality of evenly distributed contact points. This patent is incorporated herein by reference for the purpose of the desirability of the mattress and the narrow gap cell that is illustrated. However, it is understood that it is not possible to extrapolate all the teachings from non-pressurized systems and use them for large pressurized cells such as found in this invention.
U.S. Pat. Nos. 4,545,886 and 4,668,371, which are herein incorporated by reference disclose zero gap cells of the type utilized in the invention in which at least one electrode is in physical contact with an ion exchange membrane but is not embedded into or bonded to the membrane.
U.S. Pat. No. 4,448,662, which is incorporated herein by reference, discloses solid polymer chlor alkali cells containing a cation selective permionic membrane with the anodic electrocatalyst bearing on the anodic surface of the membrane which contains no electrolyte gap between the electrocatalyst bearing on the permionic membrane. The mattress of the present invention can be incorporated in the type of cell disclosed.
It is an object of the present invention to overcome the problem of overcompression of the ion exchange membrane in narrow gap and zero gap electrolysis cells which use a forced circulation of fluid that creates a pressure within the cells.
It is a further object of the invention to provide a means for selecting a mattress for large size electrolysis cells with membranes of at least about 40 ft.sup.2 that compensates for the dimensional tolerances of the electrode to electrode spacing of filter press cells.
It is a yet still further object of the invention to provide a mattress for large size electrolysis cells with sufficient resiliency to maintain all of the components in a zero gap cell in compression.
It is a yet another object of the invention to provide a mattress for large size electrolysis cells which utilize a pressurized system or a forced circulation of the anolyte and/or catholyte fluids.
It is also another object of the invention to provide as close a contact as possible of the electrodes with an intermediate membrane or diaphragm in a manner such that the membrane or diaphragm is not damaged due to excessive contact pressure.